The present invention relates to a centrifuge system and a method for controlling same, and, more particularly, to such a system and method in which the operation of the centrifuge system is sustained during power outages or shortages.
Decanting centrifuge centrifugal separator, or "centrifuges", are well known in the art and are designed to process a mixture of two constituents, usually a liquid and a solid, separating one from the other. These types of centrifuges feature a rotating bowl and a spiral screw conveyer disposed inside the bowl which rotates in the same direction as the bowl and at a different, usually less, speed. The mixture, which for the purpose of example, will be assumed to be a liquid having relatively fine solid particles entrained therein, enters the bowl and the centrifugal forces direct and hold it against the inner wall of the bowl in a "pool" while the fluid is displaced to one end portion of the bowl for discharge. The solid particles settle against the wall and are transported, or displaced, by the screw conveyor to discharge ports extending through the opposite end portion of the bowl for discharge. Typical applications of this type of centrifuge is in pulp, paper, and waste water treatments and for the removal of dirt, sand, shale, abrasive cuttings, and/or silt particles (hereinafter referred to as "solid particles") from drilling fluid after the fluid has been circulated through a drilling bit to lift the cuttings to the surface in an oil field drilling operation.
There are several parameters involved in the operation of a centrifuge, such as bowl speed and torque, conveyor speed and torque, fluid pump rate, fluid viscosity or dilution, and fluid solids content and properties. Since the operational goals of the centrifuge itself are fairly precise, it is important that the centrifuge be controlled so that its operation is optimized in response to variations in the above parameters. Also, the centrifuge itself can be operated in different modes in accordance with different design goals, such as maximum solids separation, maximum solids discard volume, etc., which requires further precise control. U.S. Pat. No. 5,857,955, assigned to the assignee of the present invention, discloses a control system which maintains precise predetermined operational modes despite variations in the various operational parameters and design goals and which includes computer programs stored on computer-readable media that can be utilized to achieve these goals.
The system disclosed in the above-identified application, as well as other systems known in the art, often use a variable frequency drive to drive the centrifuge bowl. In some of these designs, another variable frequency drive is used to drive the conveyor at a difference speed than the bowl. Variable frequency drives require constant electrical power which is properly regulated and free from power interruptions and periods of low voltage or deviations in frequency. However, when these drives are used major problems can occur when a power outage or reduction occurs (referred to as a "brownout" condition), e.g. when the voltage from the power source for the centrifuge, which is usually a generator, is reduced or terminated for a short period of time due to a malfunction of the generator, or when additional load is imposed on the generator. For example, when a brownout condition occurs even for a short period of time, the centrifuge bowl will continue rotating, albeit at a relatively lower speed, for some time thereafter due to its momentum. However, during this time the rotation of the conveyor is not being controlled. Therefore, if the bowl and the conveyor come to the same speed, the solids remaining in the bowl will not be conveyed out by the conveyor but, rather, will "pack off," or lock, the bowl to the conveyor. Even if the power is restored in a very short time, the torque, and resulting current draw, required to again create the differential speed between the bowl and the conveyor often cannot be obtained. As a result, the collected solids in the centrifuge bowl prevent subsequent separation operations and the bowl and conveyor must be cleaned either by shutting down the centrifuge and doing it manually or by an automatic clean-out cycle. These cleaning operations can be time consuming especially if done manually.
Although several options are available to accommodate brownout conditions and either sustain the separation operations or, at least, continue to expel the collected solids from the conveyor, they are less than satisfactory. For example, attempts have been made to alter the input limits to the variable frequency drive for the centrifuge bowl and/or the conveyor. However, this can result in damage to the drive. Also, large capacitor banks, or separate "stand-by" generators have been provided to come on line and provide electrical power when power loss is detected. However the capacitor banks are hazardous and the stand-by generators are both expensive and unreliable. Therefore, what is needed is a system and method according to which electrical power to a variable frequency drive is supplied during brownout conditions that is not hazardous yet relatively inexpensive and reliable.